Processes used in conditioning air from one state to another state vary widely depending on such factors as the qualities of the air to be conditioned, the temperature and humidity to which the air must be conditioned, the volume of air to be conditioned, and whether a portion of the conditioned air will be recirculated or whether 100% outside air will be used. Additionally, the qualities desired in the conditioned air vary. In some instances only the temperature of the air is important whereas in other instances the humidity of the air is more important.
The painting industry, and particularly the automobile painting industry, is one example of where the humidity as well as the temperature of the air must be maintained within an optimal range of conditions. Furthermore, in the painting industry the air is not recirculated due to harmful fumes, thus 100% outside air must be appropriately conditioned and supplied into the painting area at such a capacity to create a flow of air sufficient to remove fumes and paint overspray from the painting area. The automobile painting industry presently uses volatile organic compounds almost exclusively as solvents in its paints. The nature of solvent-based paints is such that the humidity of the air during painting is the most crucial condition that must be maintained within a specific range depending on the exact type of painting process. This creates a problem during the winter when the humidity of the outside air is below the range required for the painting process. To bring outside air during the winter into the desired range of humidity, an extensive process of heating and humidifying the outside air is performed to insure that the air will be within a predetermined range of humidity when introduced into the painting area. U.S. Pat. No. 4,940,475 to Yaeger discloses a particular type of humidifier used to humidify dry ambient air in applications such as this in the automobile painting industry. Because of the nature of solvent-based paints, precise control of the temperature of the outside air has not been a concern. Thus, conditioning of air in solvent-based painting applications has typically only been a concern during the low humidity months of winter.
However, the new development of using water-based paints in the automobile painting industry is creating new concerns about the condition of air during the summer months. The relatively new use of water-based paints is in response to environmental concerns. Water-based automobile paints have introduced an entirely new set of demands on the control of the condition of the air. Specifically, the temperature of the air, as well as the humidity, must be maintained within a specific range on a year round basis. These limits on the acceptable temperature and humidity now create a problem during the hot summer months when the outside air temperature is too high for water-based painting of automobiles. For example, one water-based painting application required the conditions of the air in the paint booth be maintained at 85.degree. F.--72% RH (relative humidity) while the outside air conditions were 95.degree. F.--38% RH in the summertime. The higher temperature of the outside air requires cooling, yet simultaneously the lower humidity ratio of the outside air necessitates humidification. Typically, the temperature and humidity conditions of the outside air change continuously during the day. This fact further complicates the conditioning process and its associated controls for painting applications using water-based paints.
Finned tube heat exchangers are used to heat or cool air being conditioned. A liquid refrigerant is passed through the tubes of the heat exchanger thus cooling the finned surface which is in direct contact with the air. The heat exchangers are arranged so that all conditioned air passes through the finned area. Usually, the air is cooled to a temperature below its dew point when water from the air condenses on the finned surface and runs off through a condensate collection and drainage arrangement. Thus, the air has been dehumidified as a result of being cooled. In the above-mentioned example of a paint booth set to operate at 85.degree. F.--92% RH, cooling coils alone cannot provide the desired temperature and humidity.
Traditional conditioning processes approach the problem of cooling the summer outside air while achieving the desired level of humidity by overcooling the air to a point below the desired temperature and then sensibly reheating the air to the desired relative humidity. However, it can readily be seen that reheating the air that was just overcooled is inherently wasteful. A conditioning method which eliminates the step of reheating would achieve substantial economies in purchased equipment as well as operating efficiencies. Thus, a need exists for an air conditioning method by which outside air can be conditioned from a high temperature to a lower temperature without the need for reheating the overcooled air to maintain the desired humidity. Furthermore, such a method should be readily controllable and capable of handling high volumes encountered in new automobile painting factories.
Therefore, an object of the present invention is to provide a method and apparatus for efficiently conditioning ambient air from a high temperature to a lower temperature while simultaneously maintaining the desired level of humidity without the need for inefficient reheating.